The present invention relates generally to communications, and more specifically to up-conversion and down-conversion, being a frequency generation device providing waveforms for use in a Virtual Local Oscillator-base system.
Many communication systems up-convert electromagnetic signals from baseband to higher frequencies for transmission, and subsequently down-convert those high frequencies back to their original frequency band when they reach the receiver, processes known as up-conversion and down-conversion (or modulation and demodulation) respectively. The original (or baseband) signal, may be, for example, data, voice or video. These baseband signals may be produced by transducers such as microphones or video cameras, be computer generated, or transferred from an electronic storage device. In general, the high frequencies provide longer range and higher capacity channels than baseband signals, and because high frequency radio frequency (RF) signals can propagate through the air, they can be used for wireless transmissions as well as hard wired or fibre channels.
All of these signals are generally referred to as radio frequency (RF) signals, which are electromagnetic signals; that is, waveforms with electrical and magnetic properties within the electromagnetic spectrum normally associated with radio wave propagation.
Wired communication systems which employ such modulation and demodulation techniques include computer communication systems such as local area networks (LANs), point to point signalling, and wide area networks (WANs) such as the Internet. These networks generally communication data signals over electrically conductive or optical fibre channels. Wireless communication systems which may employ modulation and demodulation include those for public broadcasting such as AM and FM radio, and UHF and VHF television. Private communication systems may include cellular telephone networks, personal paging devices, HF (high frequency) radio systems used by taxi services, microwave backbone networks, interconnected appliances under the Bluetooth standard, and satellite communications. Other wired and wireless systems which use RF up-conversion and down-conversion would be known to those skilled in the art.
For cellular telephones, for example, it is desirable to have transmitters and receivers (which may be referred to in combination as a transceiver) which can be fully integrated onto inexpensive, low power, integrated circuits (ICs).
As frequencies of interest in the wireless telecommunications industry (especially low-power cellular/micro-cellular voice/data personal communications systems) have risen above those used previously (approximately 900 MHz) into the 1 GHz-5 GHz spectrum, the desire to implement low-cost, power efficient receivers and transmitters has led to intensive research into the use of highly integrated designs, an increasingly important aspect for portable systems, including cellular telephone handsets.
Several attempts at completely integrated transceiver designs have met with limited success. Other RF receiver topologies exist, such as image rejection architectures, which can be completely integrated on a chip, but lack in overall performance. Although many receivers use the xe2x80x9csuper-heterodynexe2x80x9d topology, which provides excellent performance, this does not meet the desired level of integration for modern wireless systems.
Direct conversion architectures demodulate RF signals to baseband in a single step, by mixing the RF signal with a local oscillator signal at the carrier frequency of the RF signal. There is therefore no image frequency, and no image components to corrupt the signal. Direct-conversion receivers offer a high level of integratability, but also have several important problems. Hence, direct conversion receivers have thus far proved useful only for signalling formats that do not place appreciable signal energy near DC after conversion to baseband.
A typical direct conversion or homodyne receiver is shown in FIG. 1. The RF band pass filter (BPF1) 102 first filters the signal coming from the antenna 100 (this band pass filter 102 may also be a duplexer). A low noise amplifier 104 is then used to amplify the filtered antenna signal, increasing the strength of the RF signal and reducing the noise figure of the receiver.
The signal is then split into its quadrature components and down-converted to baseband in a single stage using mixers MI 110 and MQ 120, and orthogonal signals generated by local oscillator (LO) 132 and 90 degree phase shifter 130. LO 132 generates a regular, periodic signal which is tuned to the carrier frequency of the incoming wanted signal rather than a frequency offset from the carrier as in the case of the super-heterodyne receiver. The signals coming from the outputs of MI 110 and MQ 120 are now at baseband, that is, having a carrier frequency of 0 Hz. The two signals are next filtered using low pass filters LPFI 112 and LPFQ 122, are amplified by gain-controlled amplifiers AGCI 114 and AGCQ 124, and are digitized via analog to digital converters ADI 116 and ADQ 126.
Direct conversion RF receivers as illustrated in FIG. 1 have several advantages over super-heterodyne systems in terms of cost, power consumption, and level of integration, however, there are also several serious problems with direct conversion. These problems include:
noise near baseband (that is, 1/f noise) which corrupts the desired signal. The term xe2x80x9c1/f noisexe2x80x9d is used to describe a number of types of noise that are greater in magnitude at lower frequencies than at higher frequencies (typically, their magnitude increases roughly with the inverse of the signal frequency);
local oscillator (LO) leakage in the RF path that creates DC offsets in the down-converted (base-band) output signal. As the LO frequency is the same as the incoming signal being demodulated, any leakage of the LO signal through the mixers 110, 120 to their RF port will fall directly into the desired signal""s band and be down-converted to baseband as well;
local oscillator (LO) leakage into the RF path that causes desensitization. Desensitization is the reduction of desired signal gain as a result of receiver reaction to an undesired signal. The gain reduction is generally due to overload of some portion of the receiver, such as the AGC circuitry 40, 42 resulting in suppression of the desired signal because the receiver will no longer respond linearly to incremental changes in input voltage.
noise inherent to mixed-signal integrated circuits corrupts the desired signal; and
large on-chip capacitors used as high-pass filters are required to remove unwanted noise and signal energy near DC, which makes integratability expensive. These capacitors are typically placed between the mixers 114, 116 and the low pass filters 136, 138.
What is needed is a simpler and more satisfactory means of generating the signals required for certain Local Oscillator implementations.
The invention provides a simplified and effective system and method for generating a number of inputs to the mixer elements of a direct conversion (homodyne) receiver configuration which uses certain Local Oscillator techniques.
In this regard, Virtual Local Oscillators are used to provide the equivalent of a local oscillator without using frequency generators having significant spectral components (power) in the input frequency or intermediate frequencies of the receiver circuit, thereby mitigating some of the disadvantages listed above. Our co-pending PCT application (WO0117122: Improved Method and Apparatus for Up- and Down-Conversion of Radio Frequency (RF) Signals, LING, YANG (CA); WONG, LAWRENCE (CA); MANKU, TAJINDER (CA).) describes preferred implementations and relevant sections are included in the detailed description for ease of reference.
In the implementation of a system using a Virtual Local Oscillator, the circuit that generates the various time-varying signals or waveforms required to operate the VLO invention presents significant design challenges. Designs have been produced which are sufficient to serve the purpose, but they tend to be complex and have higher power consumption.
The circuit that generates the various time-varying signals or waveforms are required to have a fixed and stable phase-relationship, as well as being correctly related in terms of their power spectra relative to the operating radio (RF), intermediate (IF), and baseband frequencies of the system. Such waveforms, when applied to the mixer, permit the mixer to create internally the effect of applying the Local Oscillator signal at the required frequency.